Optically detected multipulse nuclear-quadrupole-resonance studies of trivalent praseodymium in zero and weak static magnetic fields

Optically detected multipulse nuclear quadrupole resonance is used to study trivalent praseodymium in lanthanum trifluoride in zero and weak static magnetic fields. A spin-locking spin-echo rf pulse sequence consisting of a π/2 preparation pulse, followed by repeated sequence of space t, a π/2 pulse phase shifted by 90° from the preparation pulse, and a space t. A Raman heterodyne scheme was used for optical detection. The coherence was observed to decay exponentially at a rate of 51 μsec (6.2 kHz) in zero static magnetic field. For static magnetic fields of 80 G applied along the $c_3$ axis the decay was described by time constants of 76 μsec (4.2 kHz) and 372 μsec (855 Hz). This represents a lengthening of the coherence time from a previously reported (two-pulse spin echo) 16.9 μsec (18.8 kHz) at zero field and a measured 37.5 μsec (8.5 kHz) at 80 G.